Issues with antenna repair on nets with large violations

[antonblanchard]

Antenna repair wants to insert large numbers of antenna diodes on some of the nets in Microwatt. The issue is at its worst when large met4 wires feed a minimum size gate (eg a 1x strength sky130hd cell)

An example is net _024410_:

Antenna repair wants to insert 46 diodes. A few things stand out:

• sky130hd only has a single size of antenna diode. Perhaps we need a larger one?
• The tech LEF shows antenna diodes having very little effect past 0.0225um of diffusion:

  ANTENNADIFFSIDEAREARATIO PWL ( ( 0 400 ) ( 0.0125 400 ) ( 0.0225 2609 ) ( 22.5 11600 ) ) ;

  Hard to say if this is correct. @rtimothyedwards confirms that it matches what came from Skywater, but we can't be sure if the magnitude of the effect past 0.0225um is correct.

• There was some discussion about adding dummy gate area. Do we need to add an option in OpenROAD antenna repair to do that? A combination of more gate area as well as antenna diodes might be able to fix this in a more efficient way.

Test case: antenna2.tar.gz

[vijayank88]

@antonblanchard Antenna strategy got updated with latest OpenLane. Do you had chance to re-run this?